No, the TiVo auto detects the capabilities of the device it is attached to by HDMI. You can override the settings to a lower rsolution, but this effects all output and cannot be assigned to a specific type of native signal. THe TiVo's output is fixed dependent on the settings.
When watching SD, I want it unmolested by the sat/cable box (ideally 576i but 576p will do), then my Lumagen Radiance can deinterlace and scale the signal to perfection - then automatically switch to 1080i when I switch to a HD channel.
It's very simple to implement. All of my Sky and non-Sky satellite receivers do it, all well designed bluray players do it (576i for DVD and 1080p bluray).
I see this as a design fault and hopefully TiVo will fix this, otherwise a lot of true videophiles will avoid this box.
I am not sure if I am a true videophile, but I don't really watch any SD anymore so it's not an issue for me.
I also have a prorammable remote so in theory I could set up macros to switch resolution quickly.
Also I guess a lot of radiance owners are able to afford both skyhd and TiVo, so you could permanently set TiVo to 1080i and use sky for SD.
However, it is early days so hopefully a fix will come soon.